1. Technical Field
Embodiments of the present invention described herein relate to a method of testing a periodic structure, e.g., to determine a shape of a structure having a feature repeated in one or more dimensions, the method performed such as through non-destructive testing via measurement of reflectivity or transmittance.
2. Discussion of Related Art
Generally, to fabricate electronic devices, such as semiconductor devices or display devices, processes of cleaning, thin-film growing, photolithography and thin-film etching are repeated many times to produce the consumer products. For example, in the photolithography process, a circuit of a mask where an image to be fabricated is formed and is transferred to a photosensitive material (photo resist) to form a pattern, and the pattern is used as an etch barrier to form a desired circuit on a thin film.
In the semiconductor and display devices fabricated by using the photolithography process, the desired circuit needs to be transferred to the thin film in an accurate shape in each step. This is possible based on the accuracy of the photolithography process. That is, only when the shape of a desired pattern is accurately transferred to a photo resist, and the resist layer properly functions as the etch barrier, an accurate circuit can be formed on the thin film. That is, the accurate pattern is to be formed by the photo resist before the circuit is formed on the thin film, and this can be confirmed by a testing process.
To test a pattern, there has been generally used a method of optically observing a shape of a semiconductor device using a pattern tester, for example. However, since the resolution of the pattern tester can be insufficient for determining the shapes of “nano-level” patterns which measure only a few nanometers in length. Using a pattern tester, it is difficult to perform an accurate analysis. To solve such a drawback, in the semiconductor research and production line, there has been used a method of analyzing a specific shape using equipment such as an electron microscope.
However, when an electron microscope is used, since a section of a semiconductor device is to be cut for the analysis of a shape thereof, the semiconductor device as fabricated cannot be used again. Moreover, since the measurement is to be conducted under vacuum environment, it can take an excessively long time to obtain a result of the measurement. It may also be impossible to select particular regions of a sample to be measured. Due to the aforementioned drawbacks, the electron microscope has a limit in its practical use in the production line.
To address the aforementioned drawbacks, the technology using an optical measurement method has been developed and includes, for example, a method of using an approximate expression called the Effective Medium Approximation (EMA). A calculation method using the EMA has the problem in that, since an approximation is obtained by only a volume ratio of constituent substances in a given period, regardless of a detailed shape of a structure, it never distinguishes the detailed shape of the structure. That is, since the shape of each pattern of a circuit with a periodic structure is not specifically distinguished and only the volume ratio of constituent substances in a given period is distinguished, the difference between the real structure and the measured structure is significant. Specifically, in the periodic structure, since the calculation method using the EMA cannot clarify the different periodic structures if their volume ratios are same, a new optical measurement method is really needed.